Plating bath for the electroless plating of a substrate

ABSTRACT

A plating bath for electroless plating of a substrate with nickel. The plating bath includes a nickel ion source and a stabilizing system comprising an iodate ion source and a heavy metal ion source. The substrate can be a copper or aluminum substrate.

This application is a national stage application of International Patent Application No. PCT/EP2020/060664 filed on Apr. 16, 2020, which claims priority to German Patent Application 10 2019 112 883.8, filed on May 16, 2019, which applications are hereby incorporated by reference in their entirety.

The present disclosure relates to a plating bath for electroless plating of a substrate, in particular a copper or aluminum substrate, with nickel, the plating bath comprising a nickel ion source.

Plating baths for electroless plating with nickel are known from the state of the art. Plating baths of this kind provide an alternative to galvanic metal deposition. In galvanic metal deposition, the injection of an electric current or an electric voltage into a substrate to be plated drives the deposition of the metal dissolved in a plating electrolyte.

Electroless/chemical plating can be classified into two subgroups:

-   -   The so-called autocatalytic plating process:     -   In this case, the reduction of the metal dissolved in the         electrolyte using a reducing agent dissolved in the electrolyte         is the driving force of the process.     -   The so-called immersion plating process:     -   In this case, the electrochemical series is the driving force of         the process. In this case, the metal that has a higher degree of         nobility (in other words, the nobler metal) and that is         dissolved in the electrolyte is deposited on a metal that has a         lower degree of nobility (in other words, the less noble metal)         during an electrochemical exchange reaction.

In many applications of the production of printed circuits having conductive copper tracks and areas, nickel is to be layered onto the copper in an electroless manner. For example, the electroless application of nickel on copper is often used in printed circuits in order to form a diffusion barrier for a subsequently applied gold layer.

For electroless plating of a substrate with nickel, a variety of electroless nickel plating baths are available, which generally consist of aqueous solutions containing a source of nickel ions, a reducing agent for the nickel, and a complexing agent in order to be able to operate in predefined ranges of the pH. The most commonly used baths of this kind use hypophosphite reducing agents. With these baths, phosphor and nickel are jointly deposited on the surface to be plated.

The mentioned nickel plating baths pose the problem of very low process stability and difficult and complex process control. The very low bath stability of the bath containing the electrolyte, which is due to the autocatalytic process, can be considered to be one problem of this process. Furthermore, these plating baths are very sensitive to contamination. The decomposition of the bath due to active hydrogen, which is formed during the reduction reaction, is another problematic issue of these electroless nickel plating baths. The mentioned problems have the result that the lifetime and the bath operating time are limited to few days and that tool cleaning processes which are very intricate in terms of safety are necessary at the end of a bath operating cycle.

In order to minimize the problems mentioned above, stabilizing agents which are supposed to prevent decomposition and contamination of the plating bath are known from the state of the art. For example, thiourea compounds, thiocyanate compounds, and Pb²⁺ and Bi²⁺ ion sources are stabilizing agents known from the art. However, said stabilizing agents have the disadvantage that they are highly toxic, which makes them undesirable for environmental reasons, as well. Moreover, the heavy metal salts mentioned above have been found to also tend to accumulate on a substrate to be plated. This is caused by the reduction process taking place during the plating. If these metal ions are deposited on a substrate in the course of the process, bath decomposition processes occur again, namely when the concentration of these metal ions in the plating bath drops. The thiourea compounds mentioned above are also disadvantageous. For instance, these compounds can only be used in very low concentrations (in the range of 1 ppm) since they act as what is referred to as catalytic poisons in the plating bath and can lead to a decomposition of the plating bath if their concentrations are too high. Lead salts on the other hand lead to a deterioration of the Ni deposition rate at such concentrations, for example, which leads to a low-crystalline, fine-grain consistency of the applied layer.

The object of the present disclosure is to provide a plating bath of the kind mentioned above that overcomes the disadvantages of the plating baths from the state of the art. In particular, the object of the present disclosure is to provide a plating bath for electroless plating of a substrate that remains stable as long as possible.

This object is attained by a plating bath of the kind mentioned above that comprises a stabilizing system comprising an iodate ion source and a heavy metal ion source. The plating bath according to the disclosure is an aqueous solution. Iron salts, tin salts and cadmium salts are possible heavy metal ion sources, for example. In a particularly preferred embodiment of the plating bath according to the disclosure, however, the heavy metal ion source is a copper salt, such as copper sulfate (CuSO₄ or CuSO₄.5H₂O). The use of such a combination of copper and iodate ions as a stabilizing system has surprisingly shown that this system can effect a major increase in the lifetime of a plating bath. This has been proven inter alia by the titration method described below. Furthermore, it has been found that the plating bath according to the disclosure, which contains the stabilizing system mentioned above, does not exhibit a decrease in the deposition rate of nickel on a substrate. A surface quality examination of the nickel plating, which was carried out using an optical microscope and SEM, did not reveal any differences from conventional nickel platings, either. EDX evaluations of the phosphor content in the Ni—P layer and of the Cu/Ni interface, which were examined in an FIB cross section, also indicate a similar morphological behavior of the nickel platings irrespective of whether the stabilizing system mentioned above is used or not. Moreover, it has been found that the tank containing the plating bath according to the disclosure does not show any visible residue or contamination on the tank wall after one month.

In a preferred embodiment of the plating bath according to the disclosure, the iodate ion source is potassium iodate. A combination of copper sulfate and potassium iodate has proven to be of particularly advantageous use as a stabilizing system for a plating bath for depositing nickel.

The plating bath according to the disclosure generally comprises at least one reducing agent, in particular sodium hypophosphite and/or DMAB (dimethylaminoborane), and preferably at least one complexing agent and at least one pH adjuster.

The nickel ion source is generally nickel sulfate.

Advantageously, the iodate ion source, in particular potassium iodate, has a concentration of approx. 100 μl of a 0.05 molar solution/l to approx. 400 μl of a 0.05 molar solution/l, preferably approx. 200 μl of a 0.05 molar solution/l, and the heavy metal ion source, in particular CuSO₄.5H₂O, has a concentration of approx. 20 μl of a 0.1 molar solution/l to approx. 80 μl of a 0.1 molar solution/l, preferably approx. 40 μl of a 0.1 molar solution/l. It has been found that these concentration ranges render an ideal stabilization effect without affecting the plating process. It has further been found that a negative effect on the plating process and decomposition tendencies of the plating bath can be observed at a concentration of more than 400 μl of a 0.05 molar solution/1 and 80 μl of a 0.1 molar solution/l, respectively. An ideal concentration of the iodate ion source is approx. 200 μl of a 0.05 molar solution/l. An ideal concentration of the heavy metal ion source is approx. 40 μl of a 0.1 molar solution/l.

Preferably, the plating bath according to the disclosure has a pH of approx. 3 to 5, preferably 4.4 and a temperature of approx. 80 to 90° C., preferably 85° C. These conditions have proven particularly advantageous in the plating process.

The present disclosure further relates to the use of an iodate ion source and a heavy metal ion source, in particular a copper ion source, for stabilizing a nickel plating bath.

The present disclosure further relates to a method for depositing nickel on a substrate, the method comprising the following steps:

-   -   a) treating the substrate to be plated with an acid, in         particular sulfuric acid;     -   b) activating the substrate surface using palladium;     -   c) contacting the activated substrate with a plating bath         according to any one of claims 1 to 7.

In order to deposit nickel on, for example, a copper surface from the plating bath in an electroless manner, the copper surface has to be activated first. To this end, the copper surface is contaminated with an agent having a catalytic effect for the deposition. In the case at hand, this takes places by means of palladium, in particular palladium seeds.

Generally, each of the process steps mentioned above is followed by a rinsing of the substrate with distilled water. A drying step is generally carried out at the end. In the first step, the surface of the copper substrate is cleaned and subjected to micro-etching. This step is generally carried out using diluted sulfuric acid. The polished copper surface is then activated for a subsequent plating step using palladium seeds, which produces a catalytic surface. Then, the activated substrate is introduced into the plating bath. An example of a plating bath according to the disclosure has the following parameters:

TABLE 1 Bath conditions pH 4.4 Temperature (° C.) 85   KIO₃ concentration 200 μl/l of a 0.05 molar KIO₃ solution CuSo₄•5H₂O concentration 40 μl/l of a 0.1 molar CuSO₄•5H₂O solution Ni-salt content 7 g/l Sodium hypophosphite content 15.6 g/l Other components Complexing agent; structure stabilizers (such as lead salts or bismuth salts)

The nickel plating process on a copper substrate described here is an autocatalytic process which does not involve an exchange reaction. Ni²⁺ ions are reduced to elementary nickel by a reducing agent (sodium hypophosphite in this case), the elementary nickel precipitating on the activated copper surface. Furthermore, phosphor is co-deposited in the nickel layer. In the case at hand, this takes place through catalytic partial reactions in the system. In this regard, it is to be noted that a hydrolysis of the reducing agent, sodium hypophosphite, leads to a production of active hydrogen in an atomic state. This is reflected in chemical equation (i) below.

H₂PO₂ ⁻+H₂O→H++HPO₃ ²⁻+2H  (i)

The active hydrogen produced in this reaction is most likely primarily responsible for a decomposition of the plating bath and thus for a negative impact on the bath stability.

The reaction mechanism of the stabilizing system used in the plating bath according to the disclosure can be explained by chemical equations (1), (2a), (2b) and (2c) below as follows:

Cu²⁺+2e ⁻→Cu  (1)

IO₃ ⁻ +e ⁻→I⁻  (2a)

Cu²⁺+2I⁻→Cu+I₂  (2b)

I₂+2H*→H₂+2I⁻  (2c)

The presence of copper ions in the plating bath would lead to a deposition of elementary copper on the activated metal surface. This would in turn hinder a nickel deposition. In the mechanism at hand, it is assumed that, in the presence of iodate ions, the copper ions convert the active hydrogen produced during the nickel deposition to gaseous hydrogen, which leads to an improvement of the stability of the plating bath. Based on this assumption, two different stabilizing mechanisms are possible:

-   -   1. prevention of a decomposition of the plating bath by a         “consumption” of the active hydrogen produced by the reaction of         the sodium hypophosphite mentioned above;     -   2. prevention of a bath decomposition by preventing the         occurrence of a random reaction of the nickel ions.

The production of active hydrogen in the atomic state can be explained by the following reaction chain:

H₂PO₂ ⁻+H₂O→H⁺+HPO₃ ²⁻+2H

Ni²⁺+2H→Ni↓+2H⁺

H₂PO₂ ⁻+H→P↓+OH⁻+H₂O

H₂PO₂ ⁻+H₂O→H₂↑+H⁺+HPO₃ ²⁻

The active hydrogen produced is formed during the hydrolysis of the reducing agent NaH₂PO₂. Ni²⁺ is reduced to elementary nickel in the process, H₂PO₂ ⁻ forming elementary phosphor. The following remains to be stated regarding reactions (1), (2a), (2b) and (2c):

In the presence of sodium hypophosphite, Cu²⁺ ions are reduced to elementary copper (see reaction (1)), which precipitates on the activated metal surface. At the same time, iodate ions are reduced to iodite ions (see reaction (2a)). These iodite ions in turn react with Cu²⁺ ions to form elementary copper and elementary iodine (see reaction (2b)). The iodine can now react with the active atomic hydrogen, which leads to a reproduction of iodite ions in the plating bath (see reaction (2c)). In this way, iodite ions can be used continuously to convert the produced active hydrogen to gaseous hydrogen and thereby stabilize the plating bath. With the aid of iodate ions and using a relatively low amount of copper salt, a co-deposition of copper on the Ni—P layer and the metal substrate to be plated can be prevented. This has the effect that there is only a very low risk of a copper co-deposition, which leads to a high quality of a plating. EDX analyses of the plating have shown that no copper is present in the Ni—P plating.

Furthermore, it has been found that the plating bath according to the disclosure, which contains the stabilizing system, does not exhibit a lower deposition rate of nickel on copper substrates than a comparable plating bath without said stabilizing system. In this regard, a comparative test was carried out, in which the nickel deposition rate was run with a plating bath according to the disclosure and with a plating bath without a stabilizing system. The results are illustrated in FIG. 1. Samples were run on a small scale (bath volume 1.61) and on a large scale (bath volume 1001). Both copper test chips and copper wafers having different test structures and sizes were used for this evaluation.

FIG. 1 shows: deposition rate of a Ni—P layer on copper surfaces using a plating bath with and without a stabilizing system. A deposition time of 15 minutes was selected for each sample.

As shown in FIG. 1, none of the stabilization components (copper ions and iodate ions) act as a catalytic poison. For most of the known stabilizing agents, a certain concentration must not be exceeded since an excess of a certain concentration causes a deterioration of the plating process. For instance, an average concentration of lead salts or thiourea not having a negative effect on the plating process is very low (approx. 1 ppm). In the stabilizing system of the plating bath according to the disclosure, the concentration of the components of the stabilizing system can be significantly higher.

Since no negative impact on the plating rate is observed when using the stabilizing system of the plating bath according to the disclosure, it can be assumed that there is no co-deposition of the used stabilizing components on a substrate to be plated. After all, any co-deposition of copper ions or iodate ions would have an impact on the plating rate.

Properties of the Deposited Nickel Plating:

The surface topographies of the platings were examined using an optical microscope and a scanning electron microscope. No significant differences of the surface qualities of the Ni—P layers deposited using a plating bath with and without a stabilizing system were observed. The platings have a homogenous appearance in both cases. Physical and chemical properties of the electroless nickel platings vary depending on the phosphor content in the deposited layer. An EDX analysis showed that the phosphor content in the Ni—P plating is in the range of 6% to 7%. This range is known to provide good solderability and corrosion resistance if gold is applied to the plating. The corrosion resistance is known to increase with an increasing phosphor content in the plating.

FIG. 2 shows: FIB cross sections of copper pads plated with nickel in an electroless manner, copper sulfate and potassium iodate having been used as a stabilizing system for one pad (illustration on the right) and no stabilizing agent having been used for the comparative pad (illustration on the left). The bath compositions were identical except for the stabilizing components (copper sulfate and potassium iodate). The photos show that the structural morphology of the two samples is nearly identical.

The two copper pads showed no significant differences in the two interfaces of the platings to the copper substrate. Moreover, it is to be noted that an increase in gloss and smoothness of the layer would have to be expected in the event of a co-deposition of copper on the pad to be plated. However, such effects are not found in the case at hand, which means that a co-deposition of copper can be virtually excluded.

Ultimately, the Bath Stability was Examined.

The stability of the plating baths was examined by intentionally compromising the plating baths with a PdCl₂ solution (titration method). A certain amount of PdCl₂ solution (1 ml of a 50 mg/l solution) was admixed to the plating baths during a period of 60 seconds, and the added amount was monitored throughout said period. Table 2 shows the amount of titration solution required in order to decompose the plating bath in the presence of a stabilizing system (bath no. 2) and in the absence of a stabilizing system (bath no. 1). A combination of copper sulfate and potassium iodate was used as the stabilizing system. Baths of a volume of 1.6 liters were used. As shown in Table 2, bath no. 2 requires four times the amount of PdCl₂ in order to decompose the bath.

TABLE 2 Plating bath Amount of PDCl₂ solution required (bath volume 1.6 l) for decomposition (ml) #1 (without stabilizing system) 2.5 #2 (with stabilizing system) 10

The stability tests showed high repeatability. It could be proven that the stability of electroless nickel plating baths could be significantly increased if a stabilizing system (iodate ions and copper ions in this case) is admixed, this plating system having no impact on the plating rate and the plating quality.

Examination of the Bath Tank:

For this purpose, a plating bath according to the disclosure was left in a bath tank for approx. 1 month. A visual inspection of the tank revealed that no contaminations or deposits are deposited on the tank interior or on the bottom of the tank. The same observations could be made on smaller scales (e.g., in a beaker).

Subsequently, bath samples were collected after the plating process. Thereafter, the bath tank was emptied and filled with water. Thereafter, the water was removed from the tank and what is referred to as a stripping process was performed using nitric acid. Thereafter, the nitric acid was removed from the tank, whereupon the latter was again filled with water in order to determine possible residue of stabilizers. An ICP elementary analysis of the collected bath samples revealed that no contaminating residue resulting from the components of the stabilizing system was present in the bath samples. 

1. A plating bath for electroless plating of a substrate with nickel, the plating bath comprising a nickel ion source and a stabilizing system comprising an iodate ion source and a heavy metal ion source.
 2. The plating bath according to claim 1, wherein the heavy metal ion source is a copper salt.
 3. The plating bath according to claim 1, wherein the iodate ion source is potassium iodate.
 4. The plating bath according to claim 1, further comprising at least one reducing agent.
 5. The plating bath according to claim 1, wherein the nickel ion source is nickel sulfate.
 6. The plating bath according to claim 1, wherein the iodate ion source has a concentration of approximately 100 μl of a 0.05 molar solution/l to approximately 400 μl of a 0.05 molar solution/l and the heavy metal ion source has a concentration of approximately 20 μl of a 0.1 molar solution/l to approximately 80 μl of a 0.1 molar solution/l.
 7. The plating bath according to claim 1, wherein the plating bath has a pH of approximately 3 to 5 and a temperature of approximately 80° C. to 90° C.
 8. A use of an iodate ion source and a heavy metal ion source, in particular a copper ion source, for stabilizing a nickel plating bath.
 9. A method for depositing nickel on a substrate, the method comprising the following steps: a) treating the substrate to be plated with an acid; b) activating the substrate surface using palladium; c) contacting the activated substrate with a plating bath according to claim
 1. 10. The plating bath of claim 1, wherein the substrate is a copper or aluminum substrate.
 11. The plating bath according to claim 1, wherein the heavy metal ion source is a copper sulfate.
 12. The plating bath according to claim 4, wherein the reducing agent comprises sodium hypophosphite and/or DMAB (dimethylaminoborane).
 13. The plating bath according to claim 4, wherein the reducing agent comprises at least one complexing agent and at least one pH adjuster.
 14. The plating bath according to claim 6, wherein the iodate ion source is potassium iodate.
 15. The plating bath according to claim 6, wherein the heavy metal ion source is CuSO₄.5H₂O.
 16. The plating bath according to claim 6, wherein the iodate ion source-has a concentration of approximately 200 μl of a 0.05 molar solution/l, and the heavy metal ion source has a concentration of approximately 40 μl of a 0.1 molar solution/l.
 17. The plating bath according to claim 1, wherein the plating bath has a pH of approximately 4.4, and a temperature of approximately.
 18. The method of claim 9, wherein the step of treating the substrate to be plated comprises treating the substrate with sulfuric acid. 